In the manufacture and use of thin film resistor (TFR) type printheads for use in thermal ink jet printers, it is well known to employ resistive heater elements to heat ink to boiling and thus propel the ink through an overlying orifice plate during an ink jet printing operation. The resistive heater material is typically disposed on an insulating substrate, and the individual heater elements therein are dimensionally defined by conductive trace patterns which are photolithographically formed using conventional photolithographic masking, ultraviolet exposure and etching techniques well known in the art. This thin film printhead substructure is affixed to an intermediate barrier layer and outer orifice plate, and typically the internal geometry of the barrier layer will define a plurality of ink reservoirs which are situated above and aligned with a corresponding plurality of heater resistors. For a further discussion of these types of thin film printhead structures, reference may be made to the Hewlett-Packard Journal, Vol. 38, No. 5, May 1985, incorporated herein by reference.
One conventional prior art process for defining the X-Y dimensions of these heater resistors involves first depositing successive layers of resistive and conductive material on an insulating substrate and then using a single photoresist mask to first define only the width of the conductive strips in the outer conductive pattern. This photoresist mask also serves as an etch mask for the underlying resistive layer and defines a first dimension of a resistive heater element. After etching exposed areas of the resistive area and leaving a strip of resistive material intact beneath an outer strip of conductive material, another set of masking and etching steps is used to form openings in the conductive strips to thereby establish a second dimension of the resistive heater element.
Since the first two etches in the above process use the same photoresist mask, pattern degradation occurring during the first etch step undesirably affects the quality of the second etch. Additionally, the etchant used for the bottom layer (resistive layer) attacks the freshly exposed edges of the top layer (conductor layer), giving rise to a variety of defect possiblities. The second masking and etching step which defines the second dimension of the resistive heater element as described above also suffers from a condition caused by etching the conductor layer which has undergone surface modifications. These modifications are caused by exposure to the chemicals involved in the first masking and etching step, including trace quantities of the organic residues from the photoresist material. This latter fact manifests itself in an irregular bevel quality at the resistor/conductor interface.